LED light engines are used to illuminate box and channel letter signs. In the United States of America a typical channel letter sign has a five inch can depth, which is the distance between the rear wall and the translucent cover of the channel letter. To illuminate the channel letter, an LED string light engine attaches to the rear wall and directs light forwardly towards the translucent cover. To optimize efficiency, the LEDs are spaced as far from one another as possible before any dark spots and/or overly bright spots are noticeable on the translucent cover. To minimize dark spots, the LEDs are spaced close enough to one another so that the light beam pattern from each LED overlaps the light beam pattern from adjacent LEDs by a defined amount in order to achieve a uniform appearance to the observer of the sign.
FIG. 1 depicts a schematic representation where a first LED 10 is spaced a distance (center-to-center) W from an adjacent second LED 12 in a sign 14. In this schematic representation, the LEDs 10 and 12 attach to a rear wall 16 of the sign and direct light towards a translucent cover (a typical sign only include one cover, but this schematic depiction shows two covers each at a different distance from the LEDs for illustration purposes). A first illustrative translucent cover 18 is spaced a distance D1 from the LEDs and a second illustrative translucent cover 20 is spaced a distance D2 from the LEDs, where D1 is greater than D2.
The distance W is referred to as a stroke width, which is the distance between adjacent strips, or rows, of LED light engines in the sign or channel letter. The stroke width W is a function of the LEDs' viewing angle. The LED viewing angle Θ is twice the off-axis angle β defined by the boundary at a plane where the LED's luminous intensity is some percentage of the intensity at the direct, on-axis view normal to the plane. It is desirable to space the LEDs such that the 50% intensity boundary from the first LED 10 overlaps, coincides with or is in close proximity to the 50% intensity boundary of the second LED 12. In this fashion the 50% intensities from each LED add to about 100% of the on-axis intensity for a single LED. If this relationship is maintained throughout the sign, a desired uniformity is achieved resulting in no noticeable bright spots or dark spots on the translucent cover.
Channel letters are also manufactured having a shallower can depth, some as small as one inch. For a can depth of five inches (125 mm) and a stroke width W, the viewing angle Θ required for the 50% boundary to coincide with the 50% boundary of the adjacent LED is much narrower than the viewing angle Θ required for the 50% boundary to coincide with the 50% boundary of the adjacent LED for a one inch can depth, where the stroke width W remains the same. This is because the tan β is directly proportional to the stroke width W and inversely proportional to the can depth. This is represented with reference back to FIG. 1, where it shown that the viewing angle Θ for the LEDs is appropriate for a sign where the LEDs are spaced D1 from the translucent cover. In contrast, the viewing angle is too narrow for a sign where the LEDs are spaced D2 from the translucent cover, where the spacing W remains the same between the LEDs.
Known LED light engines used to illuminate channel letters having shallower can depths (typically less than two inches) require the LEDs to be spaced very close to one another, i.e. decrease the stroke width W, to provide the desired beam pattern overlap that was discussed above. These LED systems require many LEDs to illuminate the channel letter since the LEDs must be spaced so closely together. This results in inefficiencies with regard to energy usage as well as higher costs since the LED is typically the most expensive component of the light engine.